Multi-channel, low input capacitance signal probe and probe head

ABSTRACT

A multi-channel, low input capacitance signal probe head has a housing that receives one or more substrates having input signal pads exposed on one end of the substrate. The substrate is positioned in the housing such that the signal contact pads are exposed at an open end of the housing. A removable signal contact holder mounts to the housing and supporting electrically conductive elastomer signal contacts. The holder is disposed over an open end of housing such that the elastomer signal contacts engage the input signal pads. A probe head retention member is provided for securing the multi-channel signal probe head to a device under test.

BACKGROUND OF THE INVENTION

[0001] The present invention relates generally to signal probes and moreparticularly to a multi-channel, low input capacitance signal probe andprobe head usable with measurement test instruments, such as logicanalyzers and the like.

[0002] Logic analyzers have long been used to acquire multiple signalsfrom a device under test to analyze and verify timing, detect glitches,and the like. Multi-channel signal probes couple signals to the deviceunder test from the instrument and from the device under test to theinstrument. Various types of connectors are provided on the device undertest, such as a microprocessor mother board, for connecting the signalprobes to the device being tested. Rows of square pin connectors havetraditionally been used as the interface contacts between the deviceunder test and the probes.

[0003] The increased speed of digital circuitry requires the use ofconnectors having high speed, controlled impedance transmission lines.One such connector is called a mictor connector, manufactured by TycoElectronics, Corp., Harrisburg, Pa. A mictor connector is a high speed,controlled impedance connector having a plug and closely matingreceptacle. Each plug and receptacle portion is configured for either0.025 inch or 0.050 inch center line spacing of transmission lines andcontain from 38 to 266 lines. The transmission lines are aligned inparallel rows on either side of center power ground connector. Thecenter ground connector in the plug is a corrugated planar structurethat mates with vertically positioned ground leads in the receptacle.The transmission lines in the plug and receptacle are contained inmating housings. Mictor connectors have both vertically and horizontallymounted plugs and receptacles. The ends of the transmission linesextending from the bottom of the vertically mounted plug or receptacleare bent at an angle to form contact pads for soldering to contact padson the surface of a circuit board or the like. The ends of thetransmission lines of the horizontally mounted plug or receptacle extenddirectly outward from the bottom of the plug or receptacle for solderingto contact pads formed on opposing surfaces of the circuit board or thelike at the edge of the board. The ends of the transmission lines at theother end of the housing of the plug or receptacle form electricalcontacts that mate with each other when the closely mating plug andreceptacle are connected together. In logic analyzer probingapplications, a 38 pin mictor connector is most often used. Up to 38circuit board runs of the device under test are laid out in pattern thatterminate in a pattern corresponding to the pattern of the pins on themictor connectors. The mictor receptacle is soldered to conductive padsthat terminate the runs. In most probing applications of microprocessorboards, multiple mictor connectors are mounted on the circuit board. Themulti-channel logic analyzer probe head has the mating mictor plug. Thetransmission lines of the mictor plug are electrically coupled to centerconductors of a multiple coaxial cable type ribbon cable. Electricalelements, such as resistors, may be included in the probe head toprovide electrical isolation for the device under test.

[0004] The P6434 34-channel high density probe, manufactured and sold byTektronix, Inc., Beaverton, Oreg., for use with the TLA family of logicanalyzers is an example of a logic analyzer probe using mictorconnectors. The P6434 probe head uses an edge mounted mictor connectorthat is soldered to contact pads on opposing sides of a circuit board.The circuit board has an additional row of interconnect contact padsformed on each opposing side of the circuit board that are electricallyconnected via conductive runs to the soldered contact pads of the mictorconnector. The mictor connector and circuit board are inserted into aholder that also receives two probe cables. The probe cables are ribbontype cables having multiple lead wires. The lead wires of each probecable are soldered to contact pads of a circuit board. The contact padsare electrically connected via conductive runs to another set of contactpads that match the interconnect contact pads of the mictor connectorcircuit board. The conductive runs preferably include resistiveelements. The probe cable circuit boards are positioned on the mictorconnector circuit board with electrically conductive elastomer contactselectrically connecting the contact pads on the probe cable circuitboard to the interconnect contact pads of the mictor connector circuitboard. The circuit boards and the mictor connector are secured in placein a housing made of opposing half shells that are screwed together.

[0005] There are drawbacks to using mictor connectors and similar typeconnectors, such as Samtec connectors, for high speed probingapplications. The transmission lines of the mictor connector addscapacitive loading to the device under test which affects the fidelityof the signal being acquired. The input capacitance of the mictorconnector/probe head combination can be in the range of 2 to 2.5picofarads. The mictor connectors are permanently mounted on the circuitboard, which increases the cost of board, especially when multiplemictor connectors are used. Additionally, the complexity of the deviceunder test board layout is increased because of the need to layout traceruns to each of the mictor connector, which may result in sacrificingboard space that may otherwise be used for component layout.

[0006] What is needed is a multi-channel, low input capacitance signalprobe head for devices under test that reduces the capacitive loadingassociated with previous types of probe heads using existing highdensity connectors. In addition, the multi-channel, low inputcapacitance probe head should eliminate the need for permanently mountedconnectors on circuit boards of the device under test. Further, themulti-channel, low input capacitance probe head should provideflexibility in device under test board layout. There is also a need foradapters that connect existing connectors to the new multi-channel, lowinput capacitance signal probe head and existing multi-channel probeheads to the new connecting elements on the device under test.

SUMMARY OF THE INVENTION

[0007] Accordingly, the present invention is to a multi-channel, lowinput capacitance signal probe head usable for acquiring multiplesignals from a device under test. The signal probe head has at least afirst substrate having a plurality of input signal pads formed andexposed at one end of the substrate. The substrate is positioned in ahousing having at least a first open end and a substrate support memberthat receives the substrate such that the input signal pads are exposedat the open of the housing. A removable signal contact holder mounts tothe housing and supports electrically conductive elastomer signalcontacts. The holder is disposed over the open end the housing such thatthe elastomer signal contacts engage the input signal pads. Themulti-channel, low input capacitance signal probe head is preferablyconfigured with a second substrate having a plurality of input signalpads formed and exposed at one end of the substrate. The substratesupport member receives the second substrate such that the supportmember is disposed between the first and second substrate and the inputsignal pads on the second substrate are exposed at the open end of thehousing.

[0008] The housing preferably has opposing sidewalls walls separated byopposing front and back walls with each sidewall having a latchingrecess formed therein adjacent to the open end of the housing. Thehousing has bores formed on either side of the substrate that areperpendicular to the open end of the housing. The housing is preferablyconfigured with a substrate carrier and a substrate carrier cover. Thesubstrate carrier forms the substrate support member that receives thesubstrate with the input signal pads on the substrate being exposed atone end of the carrier. The substrate carrier cover has exterior wallsforming an interior chamber that receives the substrate carrier andsubstrate with the exterior walls forming the opposing sidewalls andfront and back walls of the housing. The substrate carrier has opposingstiles and rails with the stiles and at least one rail having recessesformed on one surface thereof for receiving the substrate with the endof the substrate having the signal pads extending to the end of the railhaving the recess. The stiles of the carrier include the housing bores.The carrier may be configured to receive a second substrate having aplurality of input signal pads thereon with the input signal pads beingexposed on one end of the substrate. The stiles and the one rail haverecesses formed on the reverse side thereof for receiving the secondsubstrate with the end of the substrate having the signal pads extendingto the end of the rail having the recess.

[0009] The removable signal contact holder preferably has a planar framemember and latching members extending perpendicular from either end ofthe frame member. At least a first slot is formed in the frame memberaligned with the input signal pads on the substrate for receiving theelectrically conductive elastomer signal contacts. The latching membershave inwardly facing latching ramps with each latching ramp having aterminating ledge that engage the latching recesses in the housingsidewalls. At least a first alignment rib is formed parallel to the sloton the planar frame that engages a corresponding recess formed in thehousing. Apertures are formed on either side of the slot that arealigned with the bores in the housing.

[0010] A probe head retention member is provided for securing themulti-channel, low input capacitance signal probe head to a device undertest. The device under test is preferably a circuit board having anarray of signal contact pads on at least one surface thereofcorresponding to the electrically conductive elastomer signal contacts.Through holes are formed on either side of the array of signal contactpads. The retention member has a first configuration with attachmentmembers, in the form of threaded screws, extending through the bores inthe housing and threadably mating with retention nuts mounted to theopposite side of the circuit board from the contact pads and alignedwith the through holes. For this configuration, flanges are formed inthe removable signal contact holder adjacent to the apertures andextending in a direction opposite from the latching members. The flangesengage the through holes in the circuit board to align the elastomersignal contacts with the array of signal contacts on the circuit board.

[0011] A second configuration for the retention member has attachmentmembers, in the form of threaded screws, extending through the bores inthe housing and threadably mating with threaded apertures disposed in aretention block positioned on the opposite side of the circuit boardfrom the contact pads and aligned with the through holes in the circuitboard. The retention block has alignment flanges formed adjacent to thethreaded apertures that have an exterior surface closely mating with andextending through the through holes in the circuit board. The flangespreferably include latching members extending outward from the flangesto engage the top surface of the circuit board. The alignment flangesextending above the circuit board are closely received in second boresextending into the housing from the open end of the housing and coaxialwith the first bores. The second bores have a diameter larger than thefirst bores with notches formed in the housing adjacent to the open endthat closely receive in the latching members.

[0012] The retention block is preferably configured with an elongatedrectangular housing having exterior walls forming an interior chamberthat receives a stiffener block having the threaded apertures formedtherein. The rectangular housing has alignment flanges extending fromone of the exterior walls adjacent to the threaded apertures with theexterior surfaces of the flanges closely mating with and extendingthrough the through holes in the circuit board. The alignment flangesalso include latching members extending outward from the flanges toengage the top surface of the circuit board.

[0013] The multi-channel, low input capacitance signal probe head isused in a multi-channel, low input capacitance measurement probe forcoupling a device under test having an array of signal contact pads onat least one surface of a circuit board and through holes formed oneither side of the array of signal contact pads to a measurementinstrument. The measurement probe has a measurement probe head with atleast a first substrate having a plurality of input signal circuits andassociated input signal pads formed thereon. The input signal pads areexposed on one end of the substrate and the input signal circuits areadjacent to and electrically coupled to the input signal pads. The firstsubstrate is disposed within a housing having a substrate carrier andsubstrate carrier cover with the substrate carrier receiving thesubstrate such that the input signal pads on the substrate are exposedat one end of the carrier. The substrate carrier cover has opposingsidewalls separated by opposing front and back walls forming an openended chamber that receives the substrate carrier and substrate suchthat the input signal pads are exposed at one of the open ends of thecover. Each sidewall of the cover has a latching recess formed thereinadjacent to the open end of the housing. The multi-channel, low inputcapacitance measurement probe is preferably configured with a secondsubstrate having a plurality of input signal circuits and associatedinput signal pads formed thereon. The substrate carrier receives thesecond substrate such that the carrier is disposed between the first andsecond substrate and the input signal pads on the second substrate areexposed at the end of the carrier.

[0014] The probe head is secured to the device under test by a probehead retention member having bores formed through the substrate carrieron either side of the substrate that are perpendicular to the open endof the housing and aligned with the through holes in the circuit board.Attachment members extend through the bores in the substrate carrier andthreadably mate with threaded apertures mounted to the opposite side ofthe circuit board from the contact pads and over the through holes.

[0015] A removable signal contact holder mounts over the open end of thehousing. The contact holder has a planar frame member and latchingmembers extending perpendicular from either end of the frame member. Theframe member has at least a first slot aligned with the input signalpads on the substrate that receives electrically conductive elastomersignal contacts. The latching members have inwardly facing latchingramps with each latching ramp having a terminating ledge that engage thelatching recesses in the housing sidewalls to mount the signal contactholder over the open end the housing such that the elastomer signalcontacts engage the input signal pads. Apertures are formed on eitherside of the electrically conductive elastomer signal contacts that arealigned with the bores in the substrate carrier and the through holes onthe circuit board. The probe head is coupled to the measurementinstrument using a multiple signal lines cable having signal lines atone end electrically coupled to outputs of the input signal circuits andthe other ends of the signal lines electrically coupled to an inputconnector that is coupled to an input connector on the measurementinstrument.

[0016] A first adapter is provided to connect existing multi-channelsignal probes to the signal contact pad configuration used with themulti-channel, low capacitance signal probe of the present invention.Existing multi-channel signal probes are terminated in a connectorhaving mating plug and receptacle portions. The respective plug andreceptacle portions have high speed, controlled impedance transmissionlines disposed within respective housings. One end of the transmissionlines form contact pads at one end of the respective housings and theother end of the transmission lines form electrical contacts at theother end of the housings. The electrical contacts engage each other onmating of the plug and receptacle. The adapter includes the other of theclosely mating plug and receptacle. The contact pads of the transmissionlines are affixed to a first array of contact pads formed on the topsurface of a substrate. The bottom surface of the substrate has a secondarray of contact pads formed thereon that correspond to the signalcontact pads on the circuit board of the device under test. The contactpads on the top surface are electrically coupled to correspondingcontact pads on the bottom surface. A removable signal contact holder ispositioned adjacent to the bottom surface of the substrate and supportselectrically conductive elastomer signal contacts such that theelastomer signal contacts engage the second array of contact pads. Anadapter retention member is positioned on the opposite side of thecircuit board from the signal contact pads and has attachment members tosecure the adapter to the circuit board.

[0017] A second adapter is provided to connect the multi-channel, lowinput capacitance signal probe to a plug or receptacle of a high speed,controlled impedance connector mounted to the device under test. Thesecond adapter has a housing with opposing end walls and sidewallsforming a cavity that receives the other of the closely mating plug orreceptacle. The contact pads of the transmission lines of the closelymating plug or receptacle are exposed at one end of the housing cavityand the electrical contacts of the transmission lines are exposed at theother end of the housing. The housing has probe head retention membersformed in the sidewalls on either side of the cavity and alignmentflanges disposed adjacent to the probe head retention members thatextend upward from the sidewalls. The retention members include boresformed in the sidewalls with the bores receiving pins having a threadedaperture formed therein.

[0018] The adapter includes a substrate having apertures formedtherethrough that closely receive the alignment flanges on the housing.The substrate has first and second arrays of contact pads formed on therespective top and bottom surfaces of the substrate. The first array ofcontact pads correspond to the electrically conductive elastomer signalcontacts of the multi-channel, low input capacitance signal probe headThe second array of contact pads are affixed to the correspondingcontact pads of the transmission lines of the plug or receptacle. Thefirst array of contact pads on the top surface of the substrate areelectrically coupled to the corresponding second array of contact padson the bottom surface of the substrate via conductive runs extendingthrough the substrate.

[0019] The alignment flanges mate with corresponding bores in themulti-channel, low input capacitance signal probe head such that thesignal contact pads of the multi-channel, low input capacitance signalprobe head connect to corresponding contact pads on the top surface ofthe substrate. The threaded pins of the probe head retention membersreceive attachment members, such as threaded screws, disposed in thebores of the multi-channel, low input capacitance signal probe head tosecure the probe head to the housing.

[0020] The objects, advantages and novel features of the presentinvention are apparent from the following detailed description when readin conjunction with appended claims and attached drawings.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

[0021]FIG. 1 is a perspective view of a measurement instrument forinjecting and acquiring signals from a device under test.

[0022]FIG. 2 is a closeup plan view of the signal contact pads on thedevice under test that mate with signal contacts of the multi-channel,low input capacitance signal probe according to the present invention.

[0023]FIG. 3 is an exploded perspective view of a first embodiment ofthe multi-channel, low input capacitance signal probe head according tothe present invention.

[0024]FIG. 4 is an exploded perspective view of a second embodiment ofthe multi-channel, low input capacitance signal probe head according tothe present invention.

[0025]FIG. 5 is an exploded perspective view of the preferred embodimentof the retention block in the multi-channel, low input capacitancesignal probe head according to the present invention.

[0026]FIG. 6 is an exploded perspective view of a first adapter usablewith the configuration of signal contact pads on the device under testof the multi-channel, low input capacitance signal probe of the presentinvention and existing multi-channel signal probes.

[0027]FIG. 7 is a top perspective view of the receptacle portion of themulti-channel, controlled impedance connector used with the firstadapter.

[0028]FIG. 8 is an exploded perspective view of a second adapter usablewith the multi-channel, low input capacitance signal probe of thepresent invention and existing high speed connectors mounted on thedevice under test.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0029] Referring to FIG. 1, there is shown a perspective view of ameasurement instrument 10, such as a logic analyzer, for injecting andacquiring signals from a device under test 12. The logic analyzer may bea portable stand alone instrument or a modular system with multiplemainframes. FIG. 1 shows one type of modular logic analyzer system 10having a modular mainframe 14 with multiple slots for receiving variousmodules 16. The modules 16 include a controller module 18, one or morelogic analyzer modules and pattern generator modules 20. An optionaldigital oscilloscope module may also be included in the system. Thelogic analyzer/pattern generator modules 20 are configured for a numberof channels, such as 34, 68 102, 136 channels. Up to 680 channels may beincluded in the mainframe. Multiple mainframes may be connected togetherto produce a system having up to 8,120 channels. The signal outputs fromthe mainframe are coupled to a display device 22, such as a displaymonitor, for viewing the acquired signals from the device under test 12.

[0030] Each module has input/output connectors 24, such as thereceptacle portion of commercially available Samtec or mictorconnectors. The plug portions of the samtec connectors are connected toone end of multi-channel signal probes 26. A ribbon cable 28 extend fromthe samtec connector end of the probe to a multi-channel, low inputcapacitance signal probe head 30. The multi-channel, low inputcapacitance signal probe head 30 is attached to signal contact padsformed on the device under test 12, such as a PC microprocessor motherboard 32 containing a microprocessor 34. The device under test 12 mayalso include embedded controller boards, internet switches and the like.The device under test 12 may also contain connectors for receivingadapter cards 36 for testing various busses associated with the deviceunder test 12. These busses include, but not limited to, rapid I/O bus,PCI bus, RAMBus, gigabit ethernet and the like. Three multi-channel, lowinput capacitance signal probe heads 30 are shown connected to thedevice under test 12 but it understood that any number of multi-channelprobes 26 may be connected to device under test 12.

[0031] Referring to FIG. 2, there is shown a closeup view of the signalcontact pads 40 on the device under test 12 that mate with signalcontacts of the multi-channel, low input capacitance signal probe head30. Conductive runs 42 are laid down on the circuit board 32, 36 tocouple a bus or busses on the device under test 12 to the contact pads40. The signal contact pads 40 are formed in an array on a circuit board32, 36 with the array having two rows of contact pads in the preferredembodiment. One row of the array has four sets of three contact pads andthe other row has five sets of three contact pads. Each set of contactpads has two signal pads separated by a ground contact pad for a totalof eighteen signal contact pads. One pair of the signal contact pads areused for providing positive and negative differential clock signalleaving a total of sixteen signal contact pads used as data channels.The signal contact pads may be configured as sixteen single ended datachannels or combined in pairs as eight differential signal channels. Inthe preferred embodiment of the invention, through holes 44 are formedin the circuit board 32, 36 on either side of the array of contact pads40 for accepting a probe head retention member to be described ingreater detail below. An orientation aperture 46 may also be formed inthe circuit board 32, 36 for accepting an orientation pin on the signalprobe head 30. Alternately, the probe head retention member may includestakes extending from the circuit board that are received in themulti-channel, low input capacitance signal probe head 30.

[0032] The multi-channel, low input capacitance signal probe head 30, asshown in the exploded perspective view of FIG. 3, includes a housing 50having at least a first open end 52 and a substrate support member 54that receives at least a first substrate 56. The substrate 56 has aplurality of input signal pads 58 formed thereon that are exposed at oneend of the substrate 56. The substrate 56 is positioned in the housing50 such that the input signal pads 58 are exposed at the open end 52 ofthe housing 50. A removable signal contact holder 60 mounts to thehousing 50 and supports electrically conductive elastomer signalcontacts 62. The holder 60 is positioned over the open end 52 of thehousing 50 such that the elastomer signal contacts 62 engage the inputsignal pads 58 on the substrate. A probe head retention member 64 thathas a portion formed in the housing 50 secures the signal probe head 30to a device under test 12.

[0033] The housing 50 of the signal probe head 30 has a substratecarrier 66 and cover 68. The substrate carrier 66 is preferably formedof opposing stiles 70 and rails 72 forming an open region 74 therebetween similar in form to a window sash. At least a first set ofrecesses 76 are formed in the stiles adjacent to the open region 74. Atleast a first set of recesses 78 are also formed in the rails 72 withthe recess 80 on one of the rails matching the recesses 76 on the stiles70. The recess 82 on the other rail is sized to accept a high frequencyribbon cable 84 connected to the substrate 56. The matching recesses76,80 on the substrate carrier 66 receive the substrate 56, preferably ahybrid substrate having electrical components, such as integratedcircuits 86 and passive components 87, mounted thereon forming inputsignal circuits. In the preferred embodiment of the invention, thestiles 70 and rails 72 have recesses 76, 78 formed on both sides forreceiving opposing hybrid substrates 56. A bore 88 is formed througheach of the stiles 70 parallel to the long axis of the stiles andparallel with the substrates. The bores 88 receive attachment members 90to secure the signal probe head 30 to the circuit board 32, 36 of thedevice under test 12. Second bores 89 are formed in the stiles thatextend from the open end 52 of the housing 50 and are coaxial with thefirst bores 88. Each second bore 89 has a diameter larger than the firstbore 88 and sized to closely receive an alignment flange to be describedin greater detail below. The attachment members 90 are preferablythreaded screws that are received in threaded apertures disposed on theopposite side of the circuit board 32, 36 from the array of signalcontact pads 40. The substrate carrier 66 is preferably made of a rigidmaterial, such as a liquid crystal polymer, for transferring the columnloading on the carrier produced by the attachment member 90 to thecircuit board. The cover 68 is preferably made of a nylon plastic andhas exterior walls 92 forming a cavity 94 that closely receives thesubstrate carrier 66. The narrow opposing walls 96 of the cover haveapertures 98 formed adjacent to one end that receive latching ramps 100on a removable signal contact holder 60.

[0034] Alternately, the housing 50 may be a clam-shell type design. Thehousing has a first housing member having a base and sidewalls extendingfrom at least one side of the base forming an open ended cavity. Asupport frame may be formed around the periphery of the interior portionof the base to support the substrate off of the base. Bores may beformed in the sidewalls of the base parallel with the substrate. Thebores receive the attachment members to secure the signal probe head tothe circuit boars of the device under test. A second housing memberhaving a periphery coextensive with the shape of the first housingmember is secured to the first housing member to enclose the substratein the housing. The sidewalls of the first housing member may alsoextend from both sides of the base forming open ended cavities on bothsides of base. Support frames may be formed around the periphery of theinterior portions on both sides of the base to support the substrates onboth sides of the base. Second housing members having a peripherycoextensive with the shape of the first housing member are secured tothe first housing member to enclose the substrates in the housing.

[0035] Each hybrid substrate 56 has a plurality of input signal pads 102and ground pads 104 formed on one end surface 106 of the substrate 56corresponding to the sets of signal contact pads 40 on the device undertest 12. Each of the input signal pads 102 is electrically coupled to aninput signal circuit in the form of a buffer amplifier formed in theintegrated circuit 86 mounted on the substrate 56. The substrates 56 arepositioned in the substrate carrier 66 with the substrate circuitryfacing each other and the input signal pads 58 exposed at the end of thecarrier 66. The individual signal lines of the ribbon cable 84 areelectrically coupled to the outputs of the buffer amplifier circuitry86. With the ribbon cables 84 attached to the substrate 56 and thesubstrate 56 positioned in the carrier 66, the cover 68 is slipped overthe carrier 66 and held in place by latching ribs 108 on the carrierengaging latching apertures 110 in the cover. The input signal pads 58on the substrate 56 are then exposed at the open end of the housing 52with the ribbon cable 84 extending from the opposite end 112 of thehousing 50.

[0036] The open end 52 of the housing 50 receives the removable signalcontact holder 60 that is affixed to the housing 50. The signal contactholder 60 has a planar frame member 120 and latching members 122extending in the same direction from either end of the frame member 120.At least a first slot 124 is formed in the frame member 120 that isaligned with the end of the substrate 56 in the housing 50. A secondslot 126 is formed in the frame member 120 when the signal probe head 30includes two substrates 56 that is aligned with the end of the secondsubstrate. In the preferred embodiment of the invention, slots 124 and126 are divided into two co-linear slots. Alignment ribs (not shown) areformed in the frame member 120 that align with corresponding alignmentgrooves 128 formed in the exposed end of the substrate carrier 66. Thelatching members 122 have inwardly facing latching ramps 100 that engagethe aperture 98 in the housing 50 to secure the signal contact holder 60over the open end 52 of the housing 50. An orientation pin 130 may beformed on the opposite side of the frame member 120 from the latchingmembers 122 that is received in the orientation aperture 46 in thecircuit board 32, 36 to aid in the proper orientation of signal probehead 30. The slot or slots 124, 126 in the frame member 120 receive andsupport the electrically conductive elastomer signal contacts 62 thatengage the input signal pads 58 on the substrate or substrates 56. Theelectrically conductive elastomer signal contacts 62 are formed from asheet of electrically conductive elastomer material having a centerconductive elastomer region sandwiched between outer insulatingelastomer regions. The conductive elastomer region has gold wiresextending through the elastomer material at a 0.002 inch pitch. Such aconductive elastomer is commercially available under the name of MAFInter-connect from Shin-Etsu Polymer America, Inc. of Union City, Calif.The sheet of electrically conductive elastomer material is sliced to alength that closely fits the slot or slots 124,126 of the signal contactholder 60. The height of the elastomer material is slightly larger thanthe thickness of the frame member 120 of the holder 60 so that thedownward force of the attachment members 90 on the housing 50 compressesthe electrically conductive elastomer signal contacts 62 between thesignal contact pads 40 on the circuit board 32, 36 and the input signalpads 58 on the substrate 56. The removable signal contact holder 60 alsoincludes apertures 132 formed on either side of the electricallyconductive elastomer signal contacts 62 that are aligned with the bores88 in the housing 50.

[0037] The multi-channel, low input capacitance signal probe headreplaces the high frequency, controlled impedance connector systemcurrently in use with the electrically conductive elastomer signalcontacts 62. The elastomer signal contacts 62, in conjunction with thestructure of the signal probe head that places the input signal contacts58 at the end of the housing 50, produces a low input capacitance signalprobe head 30 having an input capacitance of approximately 0.7picofarads. Reducing the input capacitance of signal probe head from 2to 2.5 picofarads to 0.7 picofarads is essential for testing busses andcircuits operating in the gigahertz range. The electrically conductiveelastomer signal contacts 62 will loss their elasticity over timeresulting in intermittent or loss of connectivity between the signalcontact pads 40 on the device under test 12 and the input signal pads 58on the substrate. The incorporation of the removable signal contactholder 60 in the multi-channel, low input capacitance signal probe head30 also allows quick and easy replacement of the elastomer signalcontacts 62 of the signal probe head 30.

[0038] The present embodiment of the signal probe head 30 is designedfor circuit boards having a thickness of 0.090 inches or greater. Inthis embodiment, flanges 134 are formed adjacent to the apertures 132formed in the signal contact holder 60 in a direction opposite thelatching members 122. The flanges 134 closely fit the through holes 44formed in the circuit board 32, 36 on either side of the signal contactpads 40 on the circuit board. The flanges 134 help align the signalprobe head 30 onto the signal contact pads 40. The probe head retentionmember 64 includes retention nuts 136 secured to the opposite side ofthe circuit board from the signal contact pads 40. Each retention nuts136 has a threaded aperture 138 that is aligned with one of the throughholes 44 on either side of the signal contact pads 40. In the preferredembodiment, the retention nuts are PEM nuts, manufactured and sold byPenn Engineering & Manufacturing Corp., Danboro, Pa. under Part No.KF2-256. The threaded screw attachment members 90 threadably engage theretention nuts 136 to secure the signal probe head 30 to the circuitboard 32, 36.

[0039]FIG. 4 is an exploded perspective view of a second embodiment ofthe multi-channel, low input capacitance signal probe head 30 accordingto the present invention. Like elements in FIG. 3 are labeled the samein FIG. 4. The housing 50 of the second embodiment is the same as inFIG. 3 with the housing 50 having a substrate carrier 66 and cover 68.The substrate carrier 66 includes the stiles 70 and rail 72configuration with the stiles and rails having the recesses 76, 78formed therein to receive the hybrid substrate or substrates 56. Thestiles 70 include the bores 88 that receive the attachment member 90.The substrate carrier 66 is closely received in the cover 68 with thenarrow opposing walls 96 of the cover 68 having apertures 98 formedadjacent to one end that receive latching ramps 100 on a removablesignal contact holder 60. The alternative configuration of the housingpreviously described may also be used in the second embodiment.

[0040] The removable signal contact holder 60 is affixed to the housingover the open end 52 of the housing 50 having the exposed signal contactpads 58 of the hybrid substrate or substrates 56. The signal contactholder 60 is configured with the planar frame member 120 having thelatching members 122 extending in the same direction from either end ofthe frame memberl20. The electrically conductive elastomer signalcontacts 62 are received and supported in the slot or slots 124,126aligned with the ends of the substrate or substrates 56. The framemember 120 includes the alignment ribs (not shown) that are aligned withthe corresponding alignment grooves 128 formed in the exposed end of thesubstrate carrier 66. The latching members 122 have the inwardly facinglatching ramps 100 that engage the aperture 98 in the housing 50 tosecure the signal contact holder 60 over the open end 52 of the housing50. The orientation pin 130 may also be formed on the opposite side ofthe frame member 120 from the latching members 122 that is received inthe corresponding orientation aperture 46 in the circuit board 32, 36 toaid in the proper orientation of signal probe head 30. The removablesignal contact holder 60 also includes the apertures 132 formed oneither side of the electrically conductive elastomer signal contacts 62that are aligned with the bores 88 in the housing 50.

[0041] The present embodiment of the signal probe head 30 is designedfor circuit boards having a thickness of less than 0.090 inches. In thisembodiment, the flanges 134 formed adjacent to the apertures 132 in thesignal contact holder 60 are removed and replaced by alignment flanges150 extending above the circuit board 32, 36. The alignment flanges 150extend from a retention block 152 that is positioned on the oppositeside of the circuit board 32, 26 from the contact pads 40. The retentionblock 152 has threaded apertures 154 formed therein that are alignedwith the through holes 44 in the circuit board 32, 36. The alignmentflanges 150 are formed adjacent to the threaded apertures 154 and aresized to be closely receives in the through holes 44 in the circuitboard 32, 36. The flanges 150 extend above the surface of the circuitboard 32, 36 and include latching members 156 that extend outward fromthe flanges 150 and engage the top surface of the circuit board 32, 36to secure the retention block 152 to the circuit board. The signal probehead 30 is positioned on the circuit board 32, 26 with the flanges 150extending into the second bores 89 in the housing 50 to help align thesignal probe head 50 on the board. The latching members 156 of theretention block 152 are closely received in notches 158 formed adjacentto the open end 52 of the housing 50. The attachment members 90, in theform of the threaded screws positioned in the bores 88 of the housing50, threadably mate with the threaded apertures 154 in the retentionblock 152. Tightening of the threaded screws 90 in the retention block152 captures the circuit board 32, 36 between the retention block 152and the signal probe head 30 and secures the signal probe head 30 to thecircuit board 32,36.

[0042]FIG. 5 shows an exploded perspective view of the preferredembodiment of the retention block 152. The retention block 152 hasrectangular housing 160 with exterior walls 162 forming an interiorchamber 164. The housing has apertures 166 formed on one of the exteriorwalls 162 that align with the through holes 44 in the circuit board 32,36. The alignment flanges 150 extend from the exterior wall 162 adjacentto the apertures 166. The exterior surface 168 of the apertures 150closely mate with and extend through the through holes 44 in the circuitboard 32,36. The alignment flanges 150 have the outwardly extendinglatching members 156 that engage the top surface of the circuit board32, 36. A stiffener block 170, such as made of brass or other similarhard material, is positioned in the interior chamber 164 of therectangular housing 160. The stiffener block 170 has threaded apertures172 formed therein that are aligned with the apertures 166 in therectangular housing 160 that receive the attachment members 90 to securethe signal probe head 30 to the circuit board 32, 36.

[0043] Backward compatibility needs to be maintained between existingmulti-channel signal probes using mictor or similar type connectors andthe new configuration of signal contact pads 40 on the device under test12 used with the multi-channel, low input capacitance signal probe 26 ofthe present invention. There also needs to be compatibility betweenmictor or similar type connectors mounted on the device under test andthe multi-channel, low input capacitance signal probe 26 of the presentinvention. In this regard, FIG. 6 is an exploded perspective view of afirst adapter 200 usable with existing multi-channel signal probes andthe new configuration of signal contact pads 40 on the device under test12. The existing multi-channel signal probe has a probe head that isterminated with the plug portion of the high speed, controlled impedanceconnector (e.g. mictor connector or the like). The connector has plugportion that closely mates with a receptacle portion 202 (FIGS. 6 and 7)with each portion having a housing 204 enclosing the transmission lines206. As previously described, the ends of the transmission linesextending from one end of the housing in the plug portion are formed asparallel rows of edge connected contact pads coupled to electricalcircuitry in the probe head. The ends of the transmission linesextending from the other end of the housing are formed as parallel rowsof electrical contacts separated by the center power ground corrugatedplanar structure contact. The ends of the transmission lines 206extending from one end of the housing of the receptacle portion 202 areformed as parallel rows of contact pads 208 on either side of centerpower ground contacts 210. The ends of the transmission lines 206extending from the other end of the housing 204 are formed as parallelrows of electrical contacts 212 separated by the center power groundcontacts 214. The electrical contacts and the center power groundcontacts engage each other when the plug is inserted into thereceptacle. The receptacle housing 204 may be inserted into a latchinghousing 216 prior to attaching the receptacle 202 to the circuit board32, 36 of the device under test 12. While it is general practice toterminate the multi-channel signal probes with the plug portion of theconnector, the adapter of the present invention may equally beimplemented with the receptacle portion of the connector terminating thesignal probe.

[0044] The adapter 200 has the other portion of the mating receptacleportion 202 of the connector which is mounted on a substrate 220. Thesubstrate 220 has a top surface 222 on which is formed an array ofcontact pads 224 corresponding to the contact pads 208 of thetransmission lines 206 of the receptacle 202. Through holes 218 areprovided in the substrate 220 for receiving the power-ground contacts210. The contact pads 208 of the receptacle 202 are affixed to thecontact pads 224 on the substrate 220 using well known connectingtechniques, such as soldering, glueing with an electrically conductiveadhesive or the like, to electrically couple the receptacle contact padsto the substrate contact pads. The bottom surface 226 of the substrate220 has a second array of contact pads 228 corresponding to the signalcontact pads 40 on the circuit board 32, 36 of the device under test 12.Conductive runs (not shown) extend through the substrate 220 to couplethe array of contact pads 224 on the top surface 222 with the array ofcontacts pads 228 on the bottom surface 226.

[0045] The adapter is preferably configured with mounting lugs 230extending from the bottom surface 226 of the substrate 220. The lugs 230are circular in form and have threaded apertures 232 formed therein. Thelugs 230 are positioned on the substrate 220 and aligned with thethrough holes 44 in the circuit board 32, 36 adjacent to the signalcontact pads 40.

[0046] A removable signal contact holder 234 is positioned adjacent tothe bottom surface 226 of the substrate 220 that support electricallyconductive elastomer signal contacts 236. The contact holder 234 hasslots 238 formed therein that align with the second array of contactpads 228 on the bottom surface of the substrate 220. Apertures 240 areformed in the contact holder 234 that align with the lugs 230 mounted onthe bottom of the substrate 220. The slots 238 receive and support theelectrically conductive elastomer signal contacts 236 that engage thecontact pads 228 on the bottom surface of the substrate 220. Theelectrically conductive elastomer signal contacts 236 are formed fromthe same electrically conductive elastomer sheet material as used informing the electrically conductive elastomer signal contacts 62 of themulti-channel, low input capacitance signal probe head 30. The height ofthe elastomer material is slightly larger than the thickness of theholder 234 so that downward force of the adapter 200 on the circuitboard 32, 36 of the device under test 12 compresses the electricallyconductive elastomer signal contacts 236 between the signal contact pads40 on the circuit board 32, 36 and the contact pads 228 on the substrate220. The removable signal contact holder 234 is positioned on theadapter 200 by inserting the holder over the lugs 230.

[0047] An adapter retention member 242 is positioned on the oppositeside of the circuit board 32, 36 from the signal contact pads 40 of thedevice under test 12 to aid in securing the adapter 200 to the deviceunder test 12. The retention member 242 is preferably rectangular inshape having a substantially planar top surface 244 that engages thebottom surface of the circuit board 32, 36. Apertures 246 are formed inthe retention member 242 that align with the through holes 44 formed inthe circuit board 32, 36 adjacent to the signal contact pads 40. Aportion of the aperture extending from the top surface of the retentionmember is bored with a slightly larger diameter to closely receive thelugs on the adapter substrate. A portion of the aperture 246 extendingfrom the bottom surface 248 of the retention member 242 is bored with aslightly larger diameter to receive an attachment member 250 thatextends into the threaded apertures 232 of the lugs 230. The attachmentmembers 250 are preferably threaded screws that when tightened capturethe circuit board 32, 36 of the device under test 12 between theremovable signal contact holder 234 and the adapter retention member242. The shorter of the threaded screws 250 are used with circuit boardshaving a thickness of 0.090 inches or less and the longer of thethreaded screws are used for circuit boards having a thickness greaterthan 0.090 inches.

[0048]FIG. 8 is an exploded perspective view of a second adapter 300 forconnecting the multi-channel, low input capacitance signal probe orprobes 30 to a mictor or similar type connector mounted on the deviceunder test 12. The connector has closely mating plug 302 and receptacleportions with each portion having a housing 304 enclosing thetransmission lines 306. The transmission lines 306 extending from oneend of the housing 302 of the plug portion are formed as parallel rowsof contact pads on either side of center power ground contacts. The endsof the transmission lines 306 extending from the other end of thehousing 304 are formed as parallel rows of electrical contacts 308separated by the center power ground corrugated planar structure contact310. The ends transmission lines extending from one end of the housingof the receptacle portion are formed as parallel rows of contact pads oneither side of center power ground contacts. The ends of thetransmission lines extending from the other end of the housing areformed as parallel rows of electrical contacts separated by the centerpower ground contacts. The electrical contacts and the center powerground contacts engage each other when the plug is inserted into thereceptacle. While it is general practice to connect the receptacleportion of the connector on the circuit board of the device under test,the adapter of the present invention may equally be implemented with theplug portion of the connector mounted on the circuit board.

[0049] The adapter 300 has a housing 312 with opposing end walls 314 andsidewalls 316 forming a cavity 318 that receives the other of theclosely mating plug 302 or receptacle. The contact pads of thetransmission lines 306 are exposed at one end of the housing cavity 318and the electrical contacts 308 of the transmission lines are exposed atthe other end of the housing. The housing 312 has probe head retentionmembers 320 formed in the sidewalls 316 on either side of the cavity 318and alignment flanges 322 disposed adjacent to the probe head retentionmembers 320 that extend upward from the sidewalls 316. The retentionmembers 320 include bores 324 formed in the sidewalls with the boresreceiving pins 326 having a threaded aperture 328 formed therein.

[0050] The adapter includes a substrate 330 having apertures 332 formedtherethrough that closely receive the alignment flanges 322 on thehousing 312. The substrate 330 has top and bottom surfaces 334, 336 withthe top surface having a first array of contact pads (not shown)corresponding to the elastomer signal contacts 62 of the multi-channel,low input capacitance signal probe head 30. The bottom surface 336 ofthe substrate 330 has a second array of contact pads 338 disposedthereon in a pattern corresponding to the contacts pads of thetransmission lines. The contact pads of the transmission lines areaffixed to the second array of contact pads using well known connectingtechniques, such as soldering, glueing with an electrically conductiveadhesive or the like, to electrically couple the plug contact pads tothe substrate contact pads. The first array of contact pads on the topsurface of the substrate are electrically coupled to the correspondingsecond array of contact pads on the bottom surface of the substrate viaconductive runs (not shown) extending through the substrate 330.

[0051] The alignment flanges 322 mate with the corresponding bores 89 inthe multi-channel, low input capacitance signal probe head 30 such thatthe elastomer signal contacts 62 of the multi-channel, low inputcapacitance signal probe head 30 connect to corresponding contact padson the top surface 334 of the substrate 330. The threaded pins 326 ofthe probe head retention members 320 receive the attachment members 90disposed in the bores 88 of the multi-channel, low input capacitancesignal probe head 30 to secure the probe head to the housing 312.

[0052] The adapter of FIG. 8 illustrates the preferred implementation ofthe adapter that allows two multi-channel, low input capacitance signalprobe heads 30 to be connected to the adapter 300. In thisconfiguration, additional probe head retention members 320 are formed inthe sidewalls 316 adjacent to the first probe head retention members 320and alignment flanges 322 are disposed adjacent to the additional probehead retention members 320 that extend upward from the sidewalls 316.The additional alignment flanges 322 mate with the corresponding bores88 in the other multi-channel, low input capacitance signal probe head30. The other multi-channel, low input capacitance signal probe head 30is secured to the housing in the same manner as the first using theattachment members 90 previously described.

[0053] A multi-channel, low input capacitance signal probe and probehead has been described having one or more substrates with eachsubstrate having a plurality of input signal pads formed and exposed atone end of the substrate. The substrate or substrates are positioned ina housing having at least a first open end and a substrate supportmember that receives the substrate or substrates such that the inputsignal pads are exposed at the open of the housing. A removable signalcontact holder mounts to the housing and supports electricallyconductive elastomer signal contacts. The holder is disposed over theopen end the housing such that the elastomer signal contacts engage theinput signal pads. The input signal pads on the substrate areelectrically coupled to signal lines of a multiple signal lines cable.The other ends of the signal lines are electrically coupled to an inputconnector for coupling to the measurement instrument.

[0054] The multi-channel, low input capacitance signal probe attaches toa device under test having an array of signal contact pads on at leastone surface of a circuit board corresponding to the electricallyconductive elastomer signal contacts. Through holes are formed on eitherside of the array of signal contact pads for receiving a probe headretention member that secures the signal probe head to the device undertest. The probe head retention member has attachment members extendingthrough bores in the housing that threadably mate with retention nuts ora retention block having threaded apertures formed therein mounted tothe opposite side of the circuit board from the contact pads.

[0055] First and second adapters have also been described that connectexisting multi-channel signal probes having multi-channel, controlledimpedance transmission line connectors to the new configuration ofsignal contact pads on the device under test used with themulti-channel, low input capacitance signal probe of the presentinvention, and existing multi-channel, controlled impedance transmissionline connectors mounted on a device under test to the multi-channel, lowinput capacitance signal probe of the present invention. The firstadapter has one of the mating plug or receptacle portions of theconnector which is mounted on a substrate. The substrate has first andsecond arrays of contact pads formed on the respective top and bottomsurfaces of the substrate. The first array of contact pads are affixedto corresponding contact pads of the transmission lines of the plug orreceptacle. The second array of contact pads correspond to the signalcontact pads on the device under test. Conductive runs extend throughthe substrate to connect the first and second array together. Aremovable signal contact holder is positioned adjacent to the bottomsurface of the substrate. The contact holder supports electricallyconductive elastomer signal contacts that electrically couple the secondarray of contact pads to the signal contact pads on the device undertest. Mounting lugs extend from the bottom surface of the substrate thatreceive the signal contact holder. The lugs are positioned in thethrough holes formed in the circuit board of the device under test. Theadapter is secured to the device under test using an adapter retentionmember that is positioned on the opposite of the circuit board from thesignal contact pads. Threaded screw attachment members are received inthreaded apertures in the lugs that when tightened compress theelectrically conductive elastomer signal contacts between the secondarray of contact pads on the substrate and the signal contact pads onthe circuit board of the device under test and secure the adapter to thecircuit board.

[0056] The second adapter has a housing with opposing end walls andsidewalls forming a cavity that receives the other of the closely matingplug or receptacle. The contact pads of the transmission lines of theclosely mating plug or receptacle are exposed at one end of the housingcavity and the electrical contacts of the transmission lines are exposedat the other end of the housing. The housing has probe head retentionmembers formed in the sidewalls on either side of the cavity andalignment flanges disposed adjacent to the probe head retention membersthat extend upward from the sidewalls. The retention members includebores formed in the sidewalls with the bores receiving pins having athreaded aperture formed therein.

[0057] The adapter includes a substrate having apertures formedtherethrough that closely receive the alignment flanges on the housing.The substrate has first and second arrays of contact pads formed on therespective top and bottom surfaces of the substrate. The first array ofcontact pads correspond to the electrically conductive elastomer signalcontacts of the multi-channel, low input capacitance signal probe headThe second array of contact pads correspond are affixed to correspondingto the contact pads of the transmission lines of the plug or receptacle.The first array of contact pads on the top surface of the substrate areelectrically coupled to the corresponding second array of contact padson the bottom surface of the substrate via conductive runs extendingthrough the substrate.

[0058] The alignment flanges mate with corresponding bores in themulti-channel, low input capacitance signal probe head such that thesignal contact pads of the multi-channel, low input capacitance signalprobe head connect to corresponding contact pads on the top surface ofthe substrate. The threaded pins of the probe head retention membersreceive attachment members, such as threaded screws, disposed in thebores of the multi-channel, low input capacitance signal probe head tosecure the probe head to the housing.

[0059] It will be obvious to those having skill in the art that manychanges may be made to the details of the above-described embodiments ofthis invention without departing from the underlying principles thereof.The scope of the present invention should, therefore, be determined onlyby the following claims.

What is claimed is:
 1. A multi-channel, low input capacitance signalprobe head comprising: at least a first substrate having a plurality ofinput signal pads formed thereon with the input signal pads beingexposed on one end of the substrate; a housing having at least a firstopen end and a substrate support member that receives the substrate suchthat the input signal pads are exposed at the open end of the housing;and a removable signal contact holder mounted to the housing andsupporting electrically conductive elastomer signal contacts with theholder disposed over the open end the housing such that the elastomersignal contacts engage the input signal pads.
 2. The multi-channel, lowinput capacitance signal probe head as recited in claim 1 furthercomprising a second substrate having a plurality of input signal padsformed thereon with the input signal pads being exposed on one end ofthe substrate with the substrate support member receiving the secondsubstrate such that the support member is disposed between the first andsecond substrate and the input signal pads on the second substrate areexposed at the open end of the housing.
 3. The multi-channel, low inputcapacitance signal probe head as recited in claim 1 wherein the housingfurther comprises opposing sidewalls walls separated by opposing frontand back walls with each sidewall having a latching recess formedtherein adjacent to the open end of the housing, and bores formedthrough the housing on either side of the substrate that areperpendicular to the open end of the housing.
 4. The multi-channel, lowinput capacitance signal probe head as recited in claim 3 wherein theremovable signal contact holder further comprises a planar frame memberand latching members extending perpendicular from either end of theframe member with at least a first slot formed in the frame memberaligned with the input signal pads on the substrate for receiving theelectrically conductive elastomer signal contacts.
 5. The multi-channel,low input capacitance signal probe head as recited in claim 4 whereinthe latching members further comprise inwardly facing latching rampswith each latching ramp having a terminating ledge that engage thelatching recesses in the housing sidewalls.
 6. The multi-channel, lowinput capacitance signal probe head as recited in claim 4 furthercomprising at least a first alignment rib formed parallel to the slot onthe planar frame that engages a corresponding recess formed in thehousing.
 7. The multi-channel, low input capacitance signal probe headas recited in claim 4 wherein the frame member further comprisesapertures formed on either side of the slot and aligned with the boresin the housing.
 8. The multi-channel, low input capacitance signal probehead as recited in claim 7 further comprising a probe head retentionmember for securing the multi-channel signal probe head to a deviceunder test.
 9. The multi-channel, low input capacitance signal probehead as recited in claim 8 wherein the removable signal contact holderfurther comprises flanges formed adjacent to the apertures and extendingin a direction opposite from the latching members.
 10. Themulti-channel, low input capacitance signal probe head as recited inclaim 9 wherein the device under test is a circuit board having an arrayof signal contact pads on at least one surface thereof corresponding tothe electrically conductive elastomer signal contacts, and through holesformed on either side of the array of signal contact pads, the probehead retention member further comprising attachment members extendingthrough the bores in the housing and threadably mating with retentionnuts mounted to the opposite side of the circuit board from the contactpads that are aligned with the through holes to secure the signal probehead to the device under test.
 11. The multi-channel, low inputcapacitance signal probe head as recited in claim 10 wherein theattachment members comprise threaded screws.
 12. The multi-channel, lowinput capacitance signal probe head as recited in claim 8 wherein thedevice under test is a circuit board having an array of signal contactpads on at least one surface thereof corresponding to the electricallyconductive elastomer signal contacts and through holes formed on eitherside of the array of signal contact pads aligned with the bores in thehousing, the probe head retention member further comprising attachmentmembers extending through the bores in the housing and threadably matingwith threaded apertures disposed in a retention block positioned on theopposite side of the circuit board from the contact pads that arealigned with the through holes in the circuit board to secure the signalprobe head to the device under test.
 13. The multi-channel, low inputcapacitance signal probe head as recited in claim 12 wherein theattachment members comprise threaded screws.
 14. The multi-channel, lowinput capacitance signal probe head as recited in claim 12 wherein theretention block further comprises alignment flanges formed adjacent tothe threaded apertures having an exterior surface closely mating withand extending through the through holes in the circuit board.
 15. Themulti-channel, low input capacitance signal probe head as recited inclaim 14 wherein the housing further comprises second bores extendingfrom the open end of the housing and coaxial with the first bores, eachsecond bore having a diameter larger than the first bore and sized toclosely receive one of the alignment flanges extending above the circuitboard.
 16. The multi-channel, low input capacitance signal probe head asrecited in claim 14 wherein the flanges include latching membersextending outward from the flanges to engage the top surface of thecircuit board.
 17. The multi-channel, low input capacitance signal probehead as recited in claim 16 wherein the housing further comprisesnotches formed adjacent to the open end of the housing that closelyreceive in the latching members.
 18. The multi-channel, low inputcapacitance signal probe head as recited in claim 12 wherein theretention block further comprises an elongated rectangular housinghaving exterior walls forming an interior chamber that receives astiffener block having the threaded apertures formed therein.
 19. Themulti-channel, low input capacitance signal probe head as recited inclaim 18 wherein the rectangular housing further comprises alignmentflanges extending from one of the exterior walls adjacent to thethreaded apertures and having an exterior surface closely mating withand extending through the through holes in the circuit board.
 20. Themulti-channel, low input capacitance signal probe head as recited inclaim 19 wherein the housing further comprises second bores extendingfrom the open end of the housing and coaxial with the first bores, eachsecond bore having a diameter larger than the first bore and sized toclosely receiving one of the alignment flanges extending above thecircuit board.
 21. The multi-channel, low input capacitance signal probehead as recited in claim 19 wherein the alignment flanges includelatching members extending outward from the flanges to engage the topsurface of the circuit board.
 22. The multi-channel, low inputcapacitance signal probe head as recited in claim 21 wherein the housingfurther comprises notches formed adjacent to the open end of the housingthat closely receive in the latching members.
 23. The multi-channel, lowinput capacitance signal probe head as recited in claim 3 wherein thehousing further comprises: a substrate carrier forming the substratesupport member that receives the substrate with the input signal pads onthe substrate being exposed at one end of the carrier; and a substratecarrier cover having exterior walls forming an interior chamber thatreceives the substrate carrier and substrate with the exterior wallsforming the opposing sidewalls and front and back walls of the housing.24. The multi-channel, low input capacitance signal probe head asrecited in claim 23 wherein the substrate carrier further comprisesopposing stiles and rails with the stiles and at least one rail havingrecesses formed on one surface thereof for receiving the substrate withthe end of the substrate having the signal pads extending to the end ofthe rail having the recess.
 25. The multi-channel, low input capacitancesignal probe head as recited in claim 24 wherein the stiles include thehousing bores.
 26. The multi-channel, low input capacitance signal probehead as recited in claim 24 further comprising a second substrate havinga plurality of input signal pads thereon with the input signal padsbeing exposed on one end of the substrate with the stiles and the onerail having recesses formed on the reverse side thereof for receivingthe second substrate with the end of the substrate having the signalpads extending to the end of the rail having the recess.
 27. Amulti-channel, low input capacitance signal probe head system mountableon a device under test having an array of signal contact pads on atleast one surface of a circuit board and through holes formed on eitherside of the array of signal contact pads comprising: at least a firstsubstrate having a plurality of input signal pads formed thereon withthe input signal pads being exposed on one end of the substrate; ahousing having at least a first open end and a substrate support memberthat receives the substrate such that the input signal pads are exposedat the open end of the housing; a probe head retention member havingbores formed through the housing on either side of the substrate thatare perpendicular to the open end of the housing and attachment membersextending through the bores in the housing and threadably mating withthreaded apertures mounted to the opposite side of the circuit boardfrom the contact pads that are aligned with the through holes in thecircuit board to secure the signal probe head to a device under test;and a removable signal contact holder mounted to the housing andsupporting electrically conductive elastomer signal contacts with theholder having apertures formed on either side of the elastomer signalcontacts and aligned with the bores in the housing, the holder disposedover the open end the housing such that the elastomer signal contactsengage the input signal pads.
 28. The multi-channel, low inputcapacitance signal probe head system as recited in claim 27 furthercomprising a second substrate having a plurality of input signal padsformed thereon with the input signal pads being exposed on one end ofthe substrate with the substrate support member receiving the secondsubstrate such that the support member is disposed between the first andsecond substrate and the input signal pads on the second substrate areexposed at the open end of the housing.
 29. The multi-channel, low inputcapacitance signal probe head system as recited in claim 27 wherein thehousing further comprises opposing sidewalls separated by opposing frontand back walls with each sidewall having a latching recess formedtherein adjacent to the open end of the housing.
 30. The multi-channel,low input capacitance signal probe head system as recited in claim 29wherein the removable signal contact holder further comprises a planarframe member and latching members extending perpendicular from eitherend of the frame member with at least a first slot formed in the framemember aligned with the input signal pads on the substrate that receivethe electrically conductive elastomer signal contacts and the latchingmembers having inwardly facing latching ramps with each latching ramphaving a terminating ledge that engage the latching recesses in thehousing sidewalls.
 31. The multi-channel, low input capacitance signalprobe head system as recited in claim 30 further comprising at least afirst alignment rib formed parallel to the slot on the planar frame thatengages a corresponding recess formed in the housing.
 32. Themulti-channel, low input capacitance signal probe head system as recitedin claim 27 wherein the removable signal contact holder furthercomprises flanges formed adjacent to the apertures and extending in adirection opposite from the latching members.
 33. The multi-channel, lowinput capacitance signal probe head system as recited in claim 32wherein the threaded apertures of the probe head retention member areformed in retention nuts secured to the opposite side of the circuitboard over the through holes to receive the attachment members.
 34. Themulti-channel, low input capacitance signal probe head system as recitedin claim 33 wherein the attachment members comprise screws threadablymating with the retention nuts.
 35. The multi-channel, low inputcapacitance signal probe head system as recited in claim 27 wherein theprobe head retention member further comprises a retention blockpositioned on the opposite side of the circuit board from the contactpads having the threaded apertures formed therein that are aligned withthe through holes to receive the attachment members.
 36. Themulti-channel, low input capacitance signal probe head system as recitedin claim 35 wherein the attachment members comprise threaded screws. 37.The multi-channel, low input capacitance signal probe head system asrecited in claim 35 wherein the retention block further comprisesalignment flanges formed adjacent to the threaded apertures having anexterior surface closely mating with and extending through the throughholes in the circuit board with the flanges having latching membersextending outward from the flanges to engage the top surface of thecircuit board.
 38. The multi-channel, low input capacitance signal probehead system as recited in claim 37 wherein the housing further comprisessecond bores extending from the open end of the housing and coaxial withthe first bores, each second bore having a diameter larger than thefirst bore and sized to closely receive one of the alignment flangesextending above the circuit board and notches formed adjacent to theopen end of the housing that closely receive in the latching members.39. The multi-channel, low input capacitance signal probe head system asrecited in claim 35 wherein the retention block further comprises anelongated rectangular housing having exterior walls forming an interiorchamber that receives a stiffener block having the threaded aperturesformed therein.
 40. The multi-channel, low input capacitance signalprobe head system as recited in claim 39 wherein the rectangular housingfurther comprises alignment flanges extending from one of the exteriorwalls adjacent to the threaded apertures and having an exterior surfaceclosely mating with and extending through the through holes in thecircuit board with the flanges having latching members extending outwardfrom the flanges to engage the top surface of the circuit board.
 41. Themulti-channel, low input capacitance signal probe head system as recitedin claim 40 wherein the housing further comprises second bores extendingfrom the open end of the housing and coaxial with the first bores, eachsecond bore having a diameter larger than the first bore and sized toclosely receive one of the alignment flanges extending above the circuitboard and notches formed adjacent to the open end of the housing thatclosely receive in the latching members.
 42. The multi-channel, lowinput capacitance signal probe head system as recited in claim 29wherein the housing further comprises: a substrate carrier forming thesubstrate support member that receives the substrate with the inputsignal pads on the substrate being exposed at one end of the carrier;and a substrate carrier cover having exterior walls forming an interiorchamber that receives the substrate carrier and substrate with theexterior walls forming the opposing sidewalls and front and back wallsof the housing.
 43. The multi-channel, low input capacitance signalprobe head system as recited in claim 42 wherein the substrate carrierfurther comprises opposing stiles and rails with the stiles and at leastone rail having recesses formed on one surface thereof for receiving thesubstrate with the end of the substrate having the signal pads extendingto the end of the rail having the recess.
 44. The multi-channel, lowinput capacitance signal probe head system as recited in claim 43wherein the stiles include the housing bores.
 45. The multi-channel, lowinput capacitance signal probe head system as recited in claim 43further comprising a second substrate having a plurality of input signalpads thereon with the input signal pads being exposed on one end of thesubstrate with the stiles and the one rail having recesses formed on thereverse side thereof for receiving the second substrate with the end ofthe substrate having the signal pads extending to the end of the railhaving the recess.
 46. A multi-channel, low input capacitancemeasurement probe for coupling a device under test having an array ofsignal contact pads on at least one surface of a circuit board andthrough holes formed on either side of the array of signal contact padsto a measurement instrument comprising: at least a first substratehaving a plurality of input signal circuits and associated input signalpads formed thereon with the input signal pads being exposed on one endof the substrate and the input signal circuits being adjacent to andelectrically coupled to the input signal pads; a housing having asubstrate carrier and substrate carrier cover with the substrate carrierreceiving the substrate such that the input signal pads on the substrateare exposed at one end of the carrier and the substrate carrier coverhaving opposing sidewalls walls separated by opposing front and backwalls forming an open ended chamber that receives the substrate carrierand substrate such that the input signal pads are exposed at one of theopen ends of the cover with each sidewall having a latching recessformed therein adjacent to the open end of the housing; a probe headretention member having bores formed through the substrate carrier oneither side of the substrate that are perpendicular to the open end ofthe housing that are aligned with the through holes in the circuit boardand attachment members extending through the bores in the substratecarrier and threadably mating with threaded apertures mounted to theopposite side of the circuit board from the contact pads and over thethrough holes to secure the measurement probe to a device under test; aremovable signal contact holder having a planar frame member andlatching members extending perpendicular from either end of the framemember with at least a first slot formed in the frame member alignedwith the input signal pads on the substrate that receive electricallyconductive elastomer signal contacts and the latching members havinginwardly facing latching ramps with each latching ramp having aterminating ledge that engage the latching recesses in the housingsidewalls to mount the signal contact holder over the open end thehousing such that the elastomer signal contacts engage the input signalpads with apertures formed on either side of the electrically conductiveelastomer signal contacts that are aligned with the bores in thesubstrate carrier and the through holes on the circuit board; and amultiple signal lines cable having the signal lines at one endelectrically coupled to outputs of the input signal circuits and thesignal lines at the other end electrically coupled to an input connectorfor coupling to the measurement instrument.
 47. A multi-channel, lowinput capacitance measurement probe as recited in claim 46 furthercomprising at least a first alignment rib formed parallel to the slot onthe planar frame that engages a corresponding recess formed in thesubstrate carrier.
 48. The multi-channel, low input capacitancemeasurement probe as recited in claim 47 wherein the removable signalcontact holder further comprises flanges formed adjacent to theapertures and extending in a direction opposite from the latchingmembers.
 49. The multi-channel, low input capacitance measurement probeas recited in claim 46 wherein the threaded apertures of the probe headretention member are formed in retention nuts secured to the oppositeside of the circuit board over the through holes to receive theattachment members.
 50. The multi-channel, low input capacitancemeasurement probe as recited in claim 49 wherein the attachment memberscomprise screws threadably mating with the retention nuts.
 51. Themulti-channel, low input capacitance measurement probe as recited inclaim 46 wherein the probe head retention member further comprises aretention block positioned on the opposite side of the circuit boardfrom the contact pads having the threaded apertures formed therein thatare aligned with the through holes to receive the attachment members.52. The multi-channel, low input capacitance measurement probe asrecited in claim 51 wherein the attachment members comprise threadedscrews.
 53. The multi-channel, low input capacitance measurement probeas recited in claim 51 wherein the retention block further comprisesalignment flanges formed adjacent to the threaded apertures having anexterior surface closely mating with and extending through the throughholes in the circuit board with the flanges having latching membersextending outward from the flanges to engage the top surface of thecircuit board.
 54. The multi-channel, low input capacitance measurementprobe as recited in claim 53 wherein the housing further comprisessecond bores extending from the open end of the housing and coaxial withthe first bores, each second bore having a diameter larger than thefirst bore and sized to closely receive one of the alignment flangesextending above the circuit board and notches formed adjacent to theopen end of the housing that closely receive in the latching members.55. The multi-channel, low input capacitance measurement probe asrecited in claim 51 wherein the retention block further comprises anelongated rectangular housing having exterior walls forming an interiorchamber that receives a stiffener block having the threaded aperturesformed therein.
 56. The multi-channel, low input capacitance measurementprobe as recited in claim 55 wherein the rectangular housing furthercomprises alignment flanges extending from one of the exterior wallsadjacent to the threaded apertures and having an exterior surfaceclosely mating with and extending through the through holes in thecircuit board with the flanges having latching members extending outwardfrom the flanges to engage the top surface of the circuit board.
 57. Themulti-channel, low input capacitance measurement probe as recited inclaim 56 wherein the housing further comprises second bores extendingfrom the open end of the housing and coaxial with the first bores, eachsecond bore having a diameter larger than the first bore and sized toclosely receiving one of the alignment flanges extending above thecircuit board and notches formed adjacent to the open end of the housingthat closely receive in the latching members.
 58. The multi-channel, lowinput capacitance measurement probe as recited in claim 46 wherein thesubstrate carrier further comprises opposing stiles and rails with thestiles and at least one rail having recesses formed on one surfacethereof for receiving the substrate with the end of the substrate havingthe signal pads extending to the end of the rail having the recess. 59.The multi-channel, low input capacitance measurement probe as recited inclaim 58 wherein the stiles include the substrate carrier bores.
 60. Themulti-channel, low input capacitance measurement probe as recited inclaim 58 further comprising a second substrate having a plurality ofinput signal circuits and associated input signal pads thereon with theinput signal pads being exposed on one end of the substrate and theinput signal circuits being adjacent to and electrically coupled to theinput signal pads with the stiles and the one rail having recessesformed on the reverse side thereof for receiving the second substratewith the end of the substrate having the signal pads extending to theend of the rail having the recess.
 61. The multi-channel, low inputcapacitance measurement probe as recited in claim 46 further comprisinga second substrate having a plurality of input signal circuits andassociated input signal pads formed thereon with the input signal padsbeing exposed on one end of the substrate and the input signal circuitsbeing adjacent to and electrically coupled to the input signal pads withthe substrate support member receiving the second substrate such thatthe support member is disposed between the first and second substrateand the input signal pads on the second substrate are exposed at theopen end of the housing.